1. Field of the Invention
The present invention relates to a nacelle stand and to a method for building and testing a nacelle for a wind turbine using a nacelle stand, the nacelle stand being suitable for supporting a nacelle for a wind turbine, during manufacturing and assembly of the nacelle, and during transport, the nacelle comprising a main frame suitable for installation of a number of components such as, e.g., a generator, a main shaft, a hub comprising a number of blades and a yaw system, the nacelle stand having a longitudinal axis and a traverse axis and comprising connecting means/a flange, arranged for rigid connection to the yaw system arranged at the main frame of the nacelle, the nacelle further comprising a nacelle cover extending over the main frame and over other items installed on the main frame.
2. Description of Related Art
It is well known that a nacelle for a modern wind turbine is a big and heavy construction and that during construction, assembly and transport of nacelles for modern wind turbines some challenges exist.
Firstly, the nacelle needs some kind of fixture to support the nacelle during assembling and typically a steel structure designed to this purpose is used in the workshop. After assembling the nacelle, it is typically transported by truck on a flatbed trailer. In order to make a safe transport without stressing the nacelle, it will normally be hoisted onto the trailer by a crane and fixed to the trailer.
As the turbines grow larger and larger so do the nacelles and road transport becomes a bigger challenge, especially due to the height of the nacelle and the trailer. In order to still be able to transport a nacelle by truck and under bridges, special trailers are used where the nacelle is resting on a very low flatbed. From International Patent Application Publication WO 03 071130 A1 and corresponding U.S. Pat. No. 7,793,964, a solution where the nacelle connects the front and the rear end of the trailer is known, where a skid plate is installed under the nacelle having only a very small clearance to the ground. This arrangement is not attractive as the nacelle is carried at the hub and at the main frame which is not designed to carry the weight of the nacelle. By arranging the nacelle this way, unwanted stresses and distortion of the various parts of the nacelle can occur which might damage the nacelle.
From International Patent Application Publication WO 2008/000267 A1 and corresponding U.S. Pat. No. 8,087,896, a jack up solution is known where a number of hydraulic actuators are fixated to the frame of the nacelle. When activating the hydraulic actuators, a linear force is exerted and the nacelle can be raised or lowered in relation to the ground or a flatbed trailer. The document teaches that this way there is no need for a crane at the erection site as the nacelle can be unloaded without a crane. This is however not a problem as the nacelle will still need to be hoisted to the top of the tower, and thus a crane will be present at some time during installation and erection of the wind turbine. Further, the system of WO 2008/000267 A1 and U.S. Pat. No. 8,087,896 is installed at the periphery of the main frame of the nacelle and teaches that a wobbling of the main frame of the nacelle might occur which is highly unwanted. Further, nothing in the mentioned document teaches anything about handling of the nacelle during assembly and testing.
International Patent Application Publication WO 2010/066255 A1 and U.S. Patent Application Publication 2011/260,428 also address the problem of transporting a nacelle for a wind turbine. This is addressed by having a frame with two parallel arms extending in the longitudinal direction of the nacelle and by having an inclined flange for engagement with the yaw system of the nacelle. The inclination of the flange counteracts the designed inclination of the nacelle where the main shaft that carries the hub and the rotor blades typically is inclined to ensure a certain distance between the tip of the rotor blades and the tower of the wind turbine. By having the flange inclined, the load on a trailer will be lower as the top of this specific nacelle design will be more or less horizontal, and thus the complete transport will possibly have a height allowing it to pass under certain bridges. The frame is designed to slide into tracks on a trailer and can thus be transported with only a minimum of clearance to the ground.
European Patent Application EP 1 564 405 A1 and corresponding U.S. Pat. No. 7,178,406 describe a solution for fatigue and maximum load testing of structural element of a wind turbine. A very solid and absolutely as rigid as possible, test bed is disclosed where, e.g., a mainframe for a wind turbine can be installed and stress tested, e.g., during development.
The test bed comprises a reinforced concrete foundation having means for installation of a cylindrical support structure. The item to be tested is mounted to the support structure and dynamic or static loads are applied in order to stress test the construction.
The test bed described in this document is thus a test bed for use during development and certainly not a bed for supporting a nacelle during assembly of the nacelle, or for carrying out some funtionality tests during assembly, and further for supporting the nacelle all the way thru production and finally to the erection site, from where it can be returned for repeated use. Several issues of the test bed as described in EP 1 564 405 A1 and corresponding U.S. Pat. No. 7,178,406 make it un-useful, but mainly the construction comprising a very solid, heavy and large concrete foundation makes it directly impossible to make use of the test bed during assembly and also during transport of the nacelle.
As nacelles are becoming larger and larger due to the overall power output of a modern wind turbine and as the type of generators used are subject to change, the weight of a typically nacelle can easily be more than 200 metric tons and can easily have a diameter or height of up to eight meters or more. Such a nacelle cannot be manufactured, assembled and finally transported and installed without serious challenges.
None of the known solutions address the problems of assembling, testing and transporting a nacelle for a modern megawatt wind turbine on a single nacelle stand that will follow the nacelle from the early assembling steps to installation at a tower at an erection site.